<div class="left">
    <a class="li" href="./admin-course.php">课程管理</a>
    <a class="li" href="./admin-class.php">班级管理</a>
    <a class="li" href="./admin-teacher.php">教师管理</a>
    <a class="li" href="./admin-student.php">学生管理</a>
    <a class="li on" href="./admin-exam.php">试题管理</a>
    <a class="li-children">选择题</a>
    <a class="li-children">对错题</a>
    <a class="li-children">阅读器</a>
</div>